Slag Chemistry and Hot Metal Composition
Blast furnace slag forms from the gangue in the ore burden (primarily SiO₂ and Al₂O₃) plus lime and magnesia added to the burden as flux (in the sinter or as direct limestone/dolomite additions). Slag volume is typically 250–350 kg/tHM, though low-gangue ore burdens can reduce this to 180–220 kg/tHM. Slag basicity (CaO/SiO₂) is maintained at 1.05–1.25, with MgO at 6–10% to improve slag fluidity and protect the bosh and lower stack refractories.
Hot metal composition from a modern blast furnace is remarkably consistent: 4.0–4.5% C (saturated at the hearth temperature), 0.3–0.8% Si (controlled by RAFT and blast temperature), 0.2–0.5% Mn (reduced from MnO in the ore), 0.05–0.12% P (from the ore and sinter), and 0.02–0.04% S (controlled by slag basicity and temperature). Silicon content is the key controllable variable: higher RAFT and hotter blast temperatures drive more Si reduction at the tuyere level, raising Si in the metal. Silicon target is set based on downstream requirements — high Si for wire rod grades, low Si for BOF plants where it represents excess heat and lime consumption.
Tapping occurs through the taphole — a refractory-plugged opening in the hearth sidewall — using a drill to open and a mud gun to close. Hot metal flows along the runner system and is separated from slag in the iron notch or skimmer before the hot metal flows into torpedo ladles. Tap duration is 1.5–3 hours; a large BF with two tapholes taps alternately to maintain continuous drainage and prevent hearth flooding.
Cast House Operations
The cast house is where the products of the blast furnace — hot metal and slag — are periodically tapped, separated, and dispatched. For a large BF producing 10,000 t/day, tapping occurs approximately every 90–150 minutes per taphole. Large furnaces operate two or four tapholes on rotation to ensure continuous drainage and prevent hearth flooding, which can cause dangerous pressure surges.
Taphole drilling and closure: The taphole is closed between taps by a plug of taphole clay, typically 1.5–2.5 m deep. To begin a tap, an electric drill bores through the clay plug. As the tap progresses, the taphole diameter gradually increases by erosion, widening the stream. When tapping is complete, a mud gun forces fresh taphole clay into the taphole under hydraulic pressure of 200–400 bar, closing the opening. The taphole angle — typically 6–12° below horizontal — is set to penetrate deep into the deadman, ensuring the full hearth liquid level is accessible.
Runner system and iron/slag separation: Hot metal and slag flow together down the iron runner. At the skimmer — a refractory weir crossing the runner at an angle — the lighter slag (density ~2.8–3.0 g/cm³) floats over the weir while the denser iron (density ~7.0 g/cm³) passes underneath through a notch. Slag is directed to a slag pot or to a granulation plant; iron flows to torpedo ladles.
Torpedo ladles are torpedo-shaped refractory-lined rail vehicles of 300–400 t capacity. Their high thermal mass minimises heat loss during transport to the BOF shop, which may be 1–5 km away. Transit time is 30–60 minutes, with hot metal temperature loss of typically 10–25 °C. On some integrated plants, torpedo ladles double as desulphurisation vessels: magnesium or calcium carbide reagent is injected into the torpedo, reducing sulphur from a typical BF level of 0.030–0.050% S to <0.002% S before BOF charging.
Hearth liquid level monitoring: On large modern BFs, electromagnetic sensors in the hearth wall continuously measure the liquid iron level. This allows operators to detect incomplete tapping (insufficient hearth drainage) and adjust taphole practice before problems escalate. Thermocouple arrays embedded in the hearth carbon brick provide a complementary picture of hearth thermal state and erosion depth.
Key blast furnace terms
Tap each card to reveal the definition.
Key blast furnace operating parameters
| param | value |
|---|---|
| Inner volume | 2,000–5,500 m³ |
| Hot metal production | 5,000–15,000 t/day |
| Blast temperature | 1,000–1,250 °C |
| Blast oxygen enrichment | 23–28% O₂ |
| Top pressure | 1.5–3.0 bar (abs) |
| RAFT (target) | 2,000–2,300 °C |
| Coke rate | 280–420 kg/tHM |
| PCI rate | 100–220 kg/tHM |
| Slag volume | 200–350 kg/tHM |
| Slag basicity (CaO/SiO₂) | 1.05–1.25 |
| Hot metal Si | 0.3–0.8% |
| Hot metal temperature | 1,480–1,520 °C |
| Campaign life | 15–25 years |
Typical ranges for a large modern blast furnace (4,000–5,500 m³ inner volume).
Pause and think
Hot metal tapped from the blast furnace always contains 4.0–4.5% carbon — regardless of the ore type, coke rate, or operating practice. Why is this carbon content fixed at this level rather than varying with process conditions?